1. Field of the Invention
This invention relates to an image forming apparatus employing an electrophotographic system and also relates to an intermediate transfer belt used therein. More particularly, it relates to an image forming apparatus in which a toner image formed on a first image bearing member is once transferred onto an intermediate transfer belt (primary transfer) and thereafter further transferred onto a second image bearing member (secondary transfer) to obtain an image, and also relates to such an intermediate transfer belt.
2. Related Background Art
Compared with image forming apparatus in which images are transferred from a first image bearing member onto a second image bearing member stuck or attracted onto a transfer drum (Japanese Patent Application Laid-open No. 63-301960, etc.), image forming apparatus making use of an intermediate transfer member have an advantage that the second image bearing member transfer mediums are not required to be worked or controlled (e.g., grasped by grippers, attracted, and made to have a curvature) and hence second image bearing members can be selected in great variety without regard to whether they have large or small widths or lengths, including thin papers (40 g/m.sup.2) up to thick papers (200 g/m.sup.2) as exemplified by envelopes, postcards and labels.
Because of such an advantage, color copying machines and color printers making use of intermediate transfer members have been made available in the market.
Recently, however, the load placed on printers increases more and more because of environmental problems and computer networking, and there is an increasing demand for higher speed and higher performance of printers. Under such circumstances, when a conventional intermediate transfer belt formed of resin or rubber is repeatedly used while being stretched at any desired tension, electric currents may leak at its edges to cause faulty images.
More specifically any photosensitive drum must be first uniformly electrostatically charged to a predetermined polarity and potential by means of a primary charging assembly in the course of its rotation. Accordingly, as this primary charging assembly, contact type charging assemblies or internal roller type ones are chiefly put into use at present. However, charging assemblies of this type utilize an electrical discharge, which is made to occur at the gap between the charging assembly and the photosensitive drum, and hence, when any photosensitive drum has a portion with a small layer thickness, the electrical discharge tends to localize at that portion because of low resistivity, so that the surface of the photosensitive drum may deteriorate to cause a local scrape.
In particular, this phenomenon tends to occur at non-image forming regions at the edge of the photosensitive drum. This is because photosensitive drums are produced chiefly by a coating process called dipping, and is caused by a small layer thickness at one side f the photosensitive drum side from which the drum begins to be coated. Moreover, as printers are made operable at higher speed, it concurrently becomes necessary to increase the electric currents flowed to primary charging assemblies, and hence the above phenomenon may increasingly occur.
Meanwhile, the intermediate transfer belt is so designed as to have a resin or rubber material layer on its surface so that the desired performance can be exhibited. From the viewpoint of ensuring uniform conductivity and preventing leak, it is usually constituted of a low-resistance conductive elastic layer and provided thereon an outermost layer having a higher resistance than the elastic layer. However, as a result of the use of printers at a high speed or over a long period of time, the high-resistance outermost layer of the intermediate transfer belt may crack or it may come off the elastic layer at the edge having weak strength (non-image forming regions), so that this can be a starting point to a possibility of defects further extending to the inner-part image forming regions.
In particular, most image forming apparatus employing the intermediate transfer belt are provided with some position detecting means on the belt in order to prevent faulty registration (color aberration) of multi-component color images. For example, a method is available in which holes are made in non-image forming regions of the intermediate transfer belt and light is passed through the holes so that the position can be detected with a photosensor. This, however, results in a low strength of the belt around the portions having holes and, as a result of long-term service, the outermost layer may crack or may come off the elastic layer at these portions, to bring about a liability for the low-resistance conductive elastic layer to become laid bare.
Then, once the position where the outermost layer has cracked or lifted at an edge of such a belt has come in agreement with the position where the local scrape has occurred in the photosensitive member, excess electric currents flow from the elastic layer of the intermediate transfer belt. This causes a fall in the voltage applied to the intermediate transfer belt, and hence a portion from which toner is not well transferred may come into being over the whole area of the region at which the intermediate transfer belt comes into contact with the photosensitive member in the latter's longitudinal direction, so that faulty images may be formed. Moreover, excess electric currents flowed to the back electrode of the photosensitive member may cause misoperation or break of electrical control systems of electrophotographic apparatus.